The invention relates to a method for modulation of, a basic clock for digital circuits, and to a modulator for modulation of a basic clock for digital circuits. The basic clock is modulated in order to give a broader bandwith to interference which is caused by the basic clock, and hence to distribute the interference energy over additional frequencies and thus to reduce the absolute magnitudes of, the interference spikes that occur.
Patent Application (DE 198 02 065.1) which has already been submitted by the applicant, discloses a method for modulation of a basic clock for digital circuits, and a corresponding modulator, in which the basic clock is split into equidistant time intervals, and the number of intervals is varied as a function of cyclically recurring random numbers, and the intervals between adjacent switching edges are varied in this manner.
This method results either in phase modulation or frequency modulation. A disadvantage of phase modulation is that the basic clock is still at a high level with such a method and a corresponding modulator.
A disadvantage of a method with frequency modulation and corresponding modulator is that, although frequency modulation is achieved which heavily attenuates the fundamental frequency, its average over time does not, however, directly correspond to the fundamental frequency. Applications which require a stable time platform corresponding to the fundamental frequency thus cannot be operated with the modulated clock.
The object of the invention is thus to specify a method for frequency modulation of a basic clock, which emits a modulated clock which, on average, is identical to the basic clock, and to specify a corresponding modulator for carrying out the method.
For a method, the object is achieved in that, in the method, the position of a switching area ai+1 following a switching edge a1 is calculated as follows:       a          i      +      1        =            (                        a          1                +        p        -                  (                                                    N                -                1                            2                        -                          Z                              i                +                1                                              )                    )        ⁢          xe2x80x83        ⁢    mod    ⁢          xe2x80x83        ⁢    p  
where
p is the number of equidistant sections per half-cycle
N is the number of possible switching edges, with N being an odd number, and
Z is the random number.
Appropriate selection of the random numbers in -this case allows a modulated frequency f mod to be produced, whose average over a period of time corresponds to the fundamental frequency. This, for example, makes it possible to use the modulated frequency to provide an accurate timebase for, for example, a clock.
The selection of the random numbers is particularly simple if said random numbers are inverted for n cycles, after n cycles, and the inverted numbers are then used to derive the switching edges. This means that, even if the selection of the random numbers used is unfortunate, the modulated frequency on average corresponds to the fundamental frequency.
The equidistant intervals can easily be produced by passing the basic clock via delay units.
For example, it is also possible to produce the equidistant intervals by the output of a counter which is operated at a multiple of the fundamental frequency to be modulated.
Since the delay units have delay elements which are connected or disconnected individually and/or in groups, it is possible to vary the equidistant time intervals.
Changes to the operating parameters can be compensated for by calibration of the delay units.
A modulator according to the invention has an arithmetic unit which can calculate the formula from claim 1.
One particularly simple implementation of the modulator is obtained by passing the basic clock to be modulated via a number of delay units, with the various delays being provided by appropriate tapping points between the delay units.
Since the delay times of the delay units are variable, the modulator overall can be used for different fundamental frequencies and modulation levels.
An inversion apparatus for inversion of the random numbers makes it simple to select said random numbers. This inversion apparatus can be switched on after n cycles of the random numbers, and can be switched off again after a further n cycles. As long as the random numbers are inverted, the inverted random numbers are used to derive the intervals between adjacent switching edges, rather than the random numbers themselves. This means that the average clock duration of the modulated frequency is equal to the duration of the modulated basic clock, irrespective of the selection of the random numbers.